Method and apparatus for conveying precoding information in a mimo system

ABSTRACT

Method and apparatus for conveying precoding information in a control message (M) from a first node ( 400 ) to a second node ( 402 ), with information on properties of an associated wireless data transmission (D) between the first and second nodes employing spatial multiplexing and precoding for the data transmission. The first node determines precoding parameters (P) for signal transmission to the receiving node, optionally based on feedback reports (F) from the receiving node. The first node encodes control information bits in precoding information fields of the control message by means of values in TBS fields of the control message, such that the TBS field values determine the interpretation of the bits in the precoding information field(s). The control message (M) is then sent to the second node.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/919,282, filed Aug. 25, 2010, which is a 371 of InternationalApplication No. PCT/SE2008/051247, filed Nov. 3, 2008, which claims thebenefit of U.S. Provisional Application No. 61/041,964, filed Apr. 3,2008, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to a method and apparatus forsupporting wireless transmissions in a telecommunication system whenmultiple antennas are used.

BACKGROUND

In 3GPP (3^(rd) Generation Partnership Project), the packet-switchedcommunication systems LTE (Long Term Evolution) and HSPA (High SpeedPacket Access) have been specified for wireless transmission of datapackets between user terminals and base stations in a cellular/mobilenetwork. LTE and other systems generally use OFDM (Orthogonal FrequencyDivision Multiplexing) involving multiple closely spaced orthogonalsub-carriers, which is a well-known technique in the art. Thesub-carriers are further divided into timeslots where eachfrequency/timeslot combination is referred to as a resource element.

In this description, a “sending node” is a node which sends informationcarrying signals over a wireless link, and a “receiving node” is a nodewhich receives and hopefully detects those signals. In the case ofdownlink transmissions, the sending node is a base station and thereceiving node is a user terminal, and vice versa for uplinktransmissions.

The use of multiple antennas in signal sending nodes and/or signalreceiving nodes may enhance capacity, coverage and reliability in awireless communication system, e.g. by achieving increased datathroughput and/or better signal detection at the receiving node.Multiple antennas can be employed in both user terminals and basestations to enable parallel and spatially multiplexed data streams usingthe same radio channel resource, which is commonly referred to as “MIMO”(Multiple-Input, Multiple-Output).

In particular, LTE is currently being developed to support and utiliseMIMO related techniques to provide high data rates in favourable channelconditions. Other wireless communication systems that may also berelevant for the following description include WCDMA (Wideband CodeDivision Multiple Access), WiMAX, UMB (Ultra Mobile Broadband), GPRS(General Packet Radio Service) and GSM (Global System for Mobilecommunications).

In MIMO systems, spatial multiplexing is obtained by transmitting pluralparallel information carrying signals at the same time and frequencywhile the different signals are spatially separated from each other bymeans of plural transmit antennas. The number of parallel signals ordata streams being transmitted simultaneously is referred to as the“transmission rank”.

Furthermore, by adapting the signal transmission to the current channelconditions or properties, significant improvements may be achieved.“Transmission rank adaptation” is one form of such adaptation where thetransmission rank is dynamically adjusted to what the used channel iscurrently able to support. Another form of signal transmissionadaptation is “channel dependent precoding” where the phases andamplitudes of multiple parallel signals are adjusted to match thecurrent channel properties. Classical beam-forming is in fact oneexample of precoding where the signal phase of the signal from eachtransmit antenna is adjusted such that the signals add constructively atthe receiving node.

The parallel signals to be transmitted from multiple antennas form avector-valued signal. When precoding is employed, the signals areeffectively adjusted at the sending node by multiplying thevector-valued signal by a selected precoder matrix. This procedure isillustrated schematically in FIG. 1. A coding and modulation unit, notshown, takes information bits as input and basically produces a sequenceof information carrying symbol vectors in different parallel symbolstreams referred to as “layers” 100. In FIG. 1, r different layers 1-rare shown implying a transmission rank of r. Thus, the signal to betransmitted is comprised of r parallel symbol streams forming elementsof a symbol vector s, which are fed into a precoder unit 102.

In precoder unit 102, an N_(T)×r precoder matrix W is currently used toadjust the r symbol streams in vector s, where N_(T) denotes the numberof transmit antennas or antenna ports used. The r symbols in the symbolvector s are thus multiplied by the N_(T)×r precoder matrix W to producean adjusted symbol vector s′ which is converted into OFDM signals byIFFT (Inverse Fast Fourier Transform) units 104 in a well-known manner.The produced OFDM signals are then finally transmitted from the antennasor antenna ports 1−N_(T).

An N_(T)×N_(R) MIMO channel H, used between a sending node with N_(T)transmit antennas and a receiving node with N_(R) receive antennas, isgenerally represented with the N_(T)×N_(R) matrix, and the precodermatrix W is often chosen to match the properties and characteristics ofchannel H. When signals are transmitted over channel H, a receivedvector y_(k) for a certain resource element on a sub-carrier k, oralternatively a resource element k, can be modelled by the receivingnode as:

y _(k) =HWs _(k) +e _(k)  (1)

assuming no inter-cell interference and that precoder matrix W is known.The term e_(k) is modelled as a noise vector obtained by realisations ofa random process.

In channel dependent precoding, the precoder matrix may be selected atthe sending node based on information on the current channel propertiesas reported from the receiving node to the sending node in a feedbackreport. A common approach is to select the precoder matrix from apredefined set of precoder matrices, referred to as a codebook which isknown at both nodes. Codebook based precoding is generally employed bythe LTE standard.

The receiving node, typically a user terminal, detects the currentchannel properties based on measurements on signal transmissions fromthe sending node, typically a base station, and evaluates the precodermatrices in the codebook to determine the most appropriate one for usein the current conditions. The receiving node reports back to thesending node a precoder matrix that is recommended for signaltransmission, and the sending node is then able to apply a suitableprecoder matrix for the transmission, taking the recommended one intoconsideration.

The receiving node may recommend a single precoder matrix assuminglycovering a relatively large bandwidth of multiple sub-carriers allocatedfor the used channel, i.e. “wideband” precoding. Alternatively, when thechannel properties are notably different for different frequencies, itmay be beneficial to match individual frequencies of the channel andprovide a frequency-selective precoding recommendation, specifyingdifferent precoders for different sub-carriers or sub-bands of the totalbandwidth used.

As a result from the above, channel dependent precoding typicallyrequires substantial signalling support, particularly forfrequency-selective precoding schemes. In addition to theabove-described feedback signalling from the receiving node to thesending node, signalling in the opposite direction is typically alsonecessary to indicate which precoder is actually used in the signaltransmission. Hence, the sending node may not be assured it has obtaineda correct or relevant precoder report from the receiving node, and thereceiving node must also make sure which one is used in order to processthe received signals correctly.

The amount of signalling between the receiving node and the sending nodecan be reduced if the sending node merely sends a brief precoderconfirmation, indicating whether the recommended precoder(s) has beenapplied or not. Basically, a single bit can be used for this purpose,where “1” could mean that the transmitter has applied the recommendedprecoder(s), while “0” could mean that another, possibly fixed ordefault precoder is used, thereby overriding the precoderrecommendation. For example, “0” would also be signalled if the feedbackinformation could not be correctly decoded at the sending node.

However, the above precoder confirmation scheme implies that anydecoding errors in the feedback information should preferably bedetected, and the feedback information must therefore be codedaccordingly, e.g. by including a CRC (Cyclic Redundancy Check) in thereport, which increases the report size even more. An alternative tousing a fixed or default precoder scheme is to also signal a singlewideband precoder to the receiving node. Other precoder confirmationscheme have also been proposed, which are not necessary to describedhere though. Instead of explicitly signalling to the receiving nodewhich frequency-selective precoders are actually used by the sendingnode, the above precoder confirmation methods can thus be employed tosubstantially reduce the amount of overhead signalling to the receivingnode. The encoded bits, or even modulated symbols, originating from aparticular block of information bits, often called a transport block,can be referred to as a “codeword”. This term is also used in LTE todescribe the output from a specific so-called “HARQ (Hybrid AutomaticRepeat ReQuest) process” serving a particular transport block andproviding for retransmission of any erroneously decoded codeword. A HARQprocess involves various coding schemes such as turbo encoding, ratematching, interleaving etc.

A generated codeword is modulated and distributed over the antennas ofthe sending node. Further, data from plural codewords can be transmittedsimultaneously, also known as “multi-codeword transmission”. Forexample, in a sending node with four transmit antennas 1-4, a firstmodulated codeword may be mapped to antennas 1 and 2, and the nextcodeword may be mapped to antennas 3 and 4, and so forth. In the abovecontext of precoding, the codewords are first mapped to layers insteadof being mapped directly to the antennas.

In the field of multi-antenna transmissions of high data rate, aspecific feature of the prevailing channel conditions/properties is theso-called “channel rank” which indicates how many simultaneous signalsor data streams that the current channel can actually support.Basically, the channel rank can vary from one up to the least number oftransmit and receive antennas present at the sending and receivingnodes, respectively. For example, in a 4×2 MIMO system with fourtransmit antennas and two receive antennas, the maximum channel rank istwo.

Furthermore, the channel rank may vary in time, e.g. since fluctuatingparameters such as fast fading and interference typically influence thechannel properties. Moreover, the channel rank determines how manylayers, and ultimately also codewords, that can be successfullytransmitted simultaneously. Hence, if the current channel rank is onlyone when simultaneously transmitting two codewords mapping to twoseparate layers, i.e. using a transmission rank of two, the two signalscorresponding to the codewords will most likely interfere so much thatboth codewords are erroneously detected at the receiving node.

When precoding is employed, the transmission can be adapted to thechannel rank by using as many layers as the current channel rank. In asimple case, each layer is transmitted over a particular antenna.However, the number of codewords to transmit may differ from the numberof layers used, e.g. as in LTE. In that case, the codewords must bemapped onto the layers. For example, when four transmit antennas areavailable at the sending node, the maximum number of codewords islimited to two, while up to four layers can be transmittedsimultaneously over the respective antennas when the current channelrank=4. A fixed channel rank-dependent mapping of codewords onto layerscould then be used.

FIGS. 2 a-e illustrate some examples of possible mapping of codewordsonto layers for different channel ranks, and when four transmit antennasare available at a sending node. In the figures, “S/P” denotes anoperation of transforming serial signals to parallel signals, which iswell-known in the art. In these examples, the produced layers aredistributed on the four antennas 202 by a precoder unit 200, whichadjusts the symbol streams in the layers by means of a selected precodermatrix basically as described above.

In FIG. 2 a where rank=1, one codeword CW1 is mapped onto a single layerL1. In FIG. 2 b where rank=2, a first codeword CW1 is mapped onto afirst layer L1, while a second codeword CW2 is mapped onto a secondlayer L2. In FIG. 2 c where rank=2 again, one codeword CW1 isalternatively mapped onto two layers L1 and L2. In FIG. 2 d whererank=3, a first codeword CW1 is mapped onto a first layer L1, while asecond codeword CW2 is mapped onto a second layer L2 and a third layerL3. In FIG. 2 e where rank=4, a first codeword CW1 is mapped onto afirst layer L1 and a second layer L2, while a second codeword CW2 ismapped onto a third layer L3 and a fourth layer L4.

When dynamic transmission rank adaptation and channel dependentprecoding are employed for a MIMO channel, substantial amounts of MIMOrelated control information needs to be signalled from the sending nodeto the receiving node to support the precoding, as mentioned above. InLTE for example, a control channel called PDCCH (Physical DownlinkControl Channel) is used for conveying such MIMO related informationfrom a signal sending base station to a signal receiving user terminal.The PDCCH is currently configured with various information fields inwhich 16 bits are used for MIMO information, out of which 8 bits relateto precoding.

However, it is a drawback in the existing ways of conveying MIMO andprecoder related information from a sending node to a receiving node, asexemplified by the above-mentioned PDCCH, that a large signallingoverhead is required. As a result, the coverage of control signallingmay be seriously reduced, implying that control channel coverage maywell be a limiting factor in the use of MIMO. Furthermore, no efficientsupport has been provided for so-called transmission rank override inthe above-described precoder confirmation.

Transmission rank override thus means that the sending node is able tooverride the recommendation of transmission rank obtained from thereceiving node. This functionality may be useful in several situations,such as when the buffer has very limited amounts of data to send, orwhen scheduling on a significantly smaller part of the bandwidth thanwhat the single recommended “wideband” transmission rank refers to, etc.In addition, when a HARQ process is originally transmitted mapped to twolayers using transmission rank 3 or 4, that HARQ process cannotefficiently provide for retransmission if the transmission rank isreduced to 2 due to fluctuating channel properties.

Hence, it is generally a problem that the payload for conveying MIMO andprecoder related information, e.g. transmission rank indicator (TRI),precoder confirmation, explicit precoder matrix indicator (PMI), from asignal sending node to a signal receiving node is typically large andrequires substantial signalling bandwidth. Conversely, when only alimited given payload size is available for control signalling, theremay be a need to support the signalling of additional controlinformation.

SUMMARY

It is an object of the present invention to generally address theproblems outlined above. Further, it is an object to provide a solutionthat enables a reduced signalling overhead when conveying precoderrelated information from a sending node to a receiving node, and/orimproved efficiency and flexibility for spatial multiplexing. Theseobjects and others may be accomplished by a method and apparatusaccording to the attached independent claims.

According to one aspect, a method is provided in a first node forconveying precoding information in a control message to a second node,the message containing information that describes properties of anassociated wireless data transmission between the first and second nodesemploying spatial multiplexing and precoding for sending codewordscorresponding to transport blocks in the wireless data transmission. Inthis method, precoding parameters are determined for inclusion in thecontrol message to the second node. Control information bits in at leastone precoding information field of the control message are then encodedaccording to the determined precoding parameters by means of values inpayload size related TBS fields in the control message, wherein the TBSfield values are set to determine the interpretation of the controlinformation bits in the precoding information field(s). Finally, thecontrol message with the precoding information and TBS fields are sentto the second node.

According to another aspect, an apparatus is provided in a first nodefor conveying precoding information in a control message to a secondnode, the message containing information that describes properties of anassociated wireless data transmission between the first and second nodesemploying spatial multiplexing and precoding for sending codewordscorresponding to transport blocks in the wireless data transmission. Theapparatus in the first node comprises a precoding determining unitadapted to determine precoding parameters to be included in the controlmessage to the second node. The apparatus also comprises a controlmessage encoding unit adapted to encode control information bits in atleast one precoding information field of the control message accordingto the determined precoding parameters by means of values in payloadsize related TBS fields in the control message, wherein the TBS fieldvalues are set to determine the interpretation of the controlinformation bits in the precoding information field(s). The apparatusfurther comprises a control message sending unit adapted to send thecontrol message with the precoding information and TBS fields to thesecond node.

According to yet another aspect, a method is provided in a second nodefor obtaining precoding information in a control message from a firstnode, the message containing information that describes properties of anassociated wireless data transmission between the first and second nodesemploying spatial multiplexing and precoding for sending codewordscorresponding to transport blocks in the wireless data transmission. Inthis method, the control message is first received from the first nodeand includes precoding parameters determined by the first node. Theprecoding parameters are then detected by decoding control informationbits in at least one precoding information field of the control messageby means of values in payload size related TBS fields in the controlmessage, wherein the TBS field values are used to interpret the controlinformation bits in the precoding information field(s).

According to yet another aspect, an apparatus is provided in a secondnode for obtaining precoding information in a control message from afirst node, the message containing information that describes propertiesof an associated wireless data transmission between the first and secondnodes employing spatial multiplexing and precoding for sending codewordscorresponding to transport blocks in the wireless data transmission. Theapparatus in the second node comprises a control message receiving unitadapted to receive the control message including precoding parametersdetermined by the first node. The apparatus also comprises a controlmessage decoding unit adapted to detect the precoding parameters bydecoding control information bits in at least one precoding informationfield of the control message by means of values in payload size relatedTBS fields in the control message, using the TBS field values tointerpret the control information bits in the precoding informationfield(s).

Different embodiments are possible in the methods and apparatuses above.In one exemplary embodiment, bits in the precoding information field(s)of the control message imply different predefined sets of precodinginformation messages depending on the set TBS field values. In anotherembodiment, the precoding information comprises any one or both of: atransmission rank indicating the number of parallel layers or datastreams being simultaneously used for the associated data transmission,and at least one selected precoder matrix used for adapting signalstransmitted from multiple antennas at the node sending the data.

According to further embodiments, the data is transmitted from the firstnode to the second node and the precoding parameters are determinedbased on current channel properties indicated in a feedback reportreceived from the second node, and/or by the amount of retransmissionsdue to decoding errors at the second node. The second node may be aterminal and the first node may be a base station ordering the terminalto transmit uplink data according to the conveyed precoding information.

In other embodiments, the TBS fields may indicate a payload size paircorresponding to the payload size of a first transport block and asecond transport block. The payload size pair may then be set as (TBS,0) indicating that a first codeword is enabled and transmitted with sizeTBS while a second codeword is disabled, or as (TBS1, TBS2) indicatingthat two codewords are enabled and transmitted simultaneously with sizeTBS1 and TBS2, respectively. The payload size pair may determine theinterpretation of precoder information bits in the control message tosupport transmission rank override for precoder confirmation whenfrequency-selective precoding is employed.

The control message could be a PDCCH message with precoding relatedinformation fields, the precoding related information corresponding to a“Rank Indicator RI” or “Transmission Rank Indicator TRI”, a “PrecoderMatrix Indicator PMI”, and/or precoder confirmation, wherein these partsin the precoding related information are jointly encoded.

According to further embodiments, the TBS field values are used toindicate a HARQ process to codeword mapping according to (TBS, 0)indicating that HARQ process 1 is mapped to codeword 1 which istransmitted, or (0, TBS) indicating that HARQ process 2 is mapped tocodeword 1 which is transmitted, or (TBS1, TBS2) indicating thatcodewords 1 and 2 are both transmitted. A fixed HARQ process to codewordmapping may also be used where the TBS field values are used accordingto (TBS, 0) indicating that codeword 1 is transmitted, or (0, TBS)indicating that codeword 2 is transmitted, or (TBS1, TBS2) indicatingthat codewords 1 and 2 are both transmitted.

Further possible features and benefits of the present invention will beexplained in the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail by means of exemplaryembodiments and with reference to the accompanying drawings, in which:

FIG. 1 illustrates a precoding procedure in a signal sending node usingmultiple transmit antennas, according to the prior art.

FIGS. 2 a-e illustrate some examples of how codewords are mapped ontolayers for different channel ranks using four transmit antennas at asending node, according to the prior art.

FIG. 3 is a flow chart illustrating a procedure for conveying precoderrelated information from a signal sending node to a signal receivingnode, in accordance with one embodiment.

FIG. 4 is a block diagram illustrating a signal sending node in moredetail, in accordance with further embodiments.

FIG. 5 is a table illustrating how fields in a PDCCH message can be usedto encode precoder related information, in accordance with anotherembodiment.

DETAILED DESCRIPTION

The present invention can be used to reduce the signalling overheadneeded for precoder signalling, and/or to provide improved flexibilityfor conveying precoder related information, as follows. It can also beused to utilise any available limited signalling space more efficiently,e.g. within prevailing header size restrictions dictated by any standardprotocol(s) used.

It is assumed that one node is obliged to send control information toanother node that describes the properties of an associated wirelessdata transmission between the nodes, and that the control informationshould include fields with information on the payload size of thisassociated data transmission. These fields are generally denoted“payload size related fields” in this description.

Briefly described, existing payload size related fields in a controlmessage conveyed from a first node to a second node, carryinginformation on the payload size of an associated data transmission thatthe control message refers to, are utilised to also control or dictatethe interpretation of precoding information provided in one or morespecific fields of the control message. Thus, in addition to determiningthe payload size of transmitted transport blocks, values in the payloadsize related fields are also used to encode information bits in theprecoding information field(s) according to precoding parametersselected for data transmission. The bits communicated in the precodinginformation field(s) of the control message will thus imply differentpredefined sets of precoding information messages depending on thevalues communicated in the payload size related fields of that message,thereby providing better efficiency and flexibility without increasingthe overall signalling overhead, which will be described in more detailbelow by way of exemplary embodiments.

In the above-mentioned PDCCH message, the payload size related fieldsare referred to as “TBS (Transport Block Size) fields” which term mayrepresent any payload size related fields that can be used forimplementing the invention. For most payload sizes in LTE, the fieldsreserved for resource allocation and MCS (Modulation and Coding Scheme)jointly determine the payload size of a transport block. Sometimesadditional fields are involved as well, such as e.g. the redundancyversion (RV) field.

In any case, plural fields in the control message may jointly determinethe payload size of a transport block in an associated transmission, andthese fields could hence be mapped to a certain TBS value. The presentinvention may thus be generally used to exploit the presence of a TBSvalue per transport block, possibly inferred or implied from otherfields, without relying on exactly how these TBS values are derived,which is outside the scope of this invention. Hence the term “TBS field”should not necessarily be interpreted literally as it may also refer toan implicit or inferred TBS value.

Furthermore, the present invention is not limited to the terminology ofTBS fields. It should also be noted that conventional channel coding ofthe control message may further be executed, in addition to the aboveencoding of information bits in the precoding information field(s).Thus, the encoding of precoding information bits by means of TBS fieldsor the equivalent as described herein, should not be confused with anyconventional channel coding or the like.

The present invention enables that information entities in the precodinginformation field(s) of the control message, such as transmission rankindicator (TRI), precoder confirmation, explicit precoder matrixindicator (PMI), can be jointly encoded. By utilizing the values in thepayload size related TBS fields in this way, further compression of thesignalling overhead incurred by the control information can be achieved.When using this solution in practice, support for the above-describedfunctionality of transmission rank override for precoder confirmation aswell as transmission of a single codeword over two-layers, can be addedwithout requiring any extra signalling bandwidth.

A standard PDCCH message is configured to contain a precodinginformation field previously called the “HARQ swap flag field” andcurrently referred to as the “transport block to codeword swap flag”.This flag is normally used to indicate the mapping of HARQ processes tocodewords, e.g. to indicate which codeword to retransmit. In that case,the TBS fields in the PDCCH message can be further utilised to indicatethe codeword to be retransmitted such that the original HARQ swap flagfield becomes redundant and can be omitted, thereby further compressingthe precoding information in the PDCCH message.

FIG. 3 is a flow chart illustrating a procedure in a first node forconveying precoding information in a control message to a second node,the control message containing information that describes properties ofan associated wireless data transmission employing spatial multiplexingand precoding for sending codewords corresponding to transport blocks.The first node may be a data sending node and the second node may be adata receiving node, or vice versa. Although precoding information istypically conveyed from a data sending node to a data receiving node, itis also possible to convey such precoding information from a datareceiving node to a sending data node, e.g. in the case when the firstnode is a base station that orders a terminal, being the second node, totransmit uplink data according to the conveyed precoding information,e.g. with a certain precoder and transmission rank. In that case, thebase station is the first node receiving data, and the terminal is thesecond node sending data.

It is assumed that the used control message is configured to containfields reserved for information on the size of communicated transportblocks or the like, i.e. the above-mentioned TBS fields. However, theterm “TBS” is used in this description to represent any parameter or setof parameters indicating the size of data transport units in general.Further, the term “codeword” is used to represent any predefined set ofencoded data bits. The control message is further configured to alsocontain one or more precoding information fields.

In a first optional step 300, assuming that the first node is a datasending node and the second node is a data receiving node, a feedbackreport referring to the current channel properties may be received fromthe second node, e.g. according to the above-described mechanism ofsignal transmission adaptation by channel dependent precoding.Typically, a receiving node is obliged to frequently provide suchfeedback reports to the sending node in order to keep the channelproperties up-to-date therein. A transmission rank and one or moreprecoder matrices may be recommended in the feedback report, asmentioned above. However, this step may be omitted depending on theimplementation.

In a next step 302, precoding parameters are determined or selected foran associated data transmission between the two nodes, which maycomprise a transmission rank and/or a predefined precoder matrix. Theprecoding parameters can be determined optionally based on the receivedfeedback report, although other basis may additionally or alternativelybe used, e.g. the amount of retransmissions of data due to data decodingerrors at the receiving node.

As described above, a receiving node may recommend certain precodingparameters in a feedback report, and the sending node will then eitherconfirm or override the recommendation. Further, if no or insufficientbasis for determining precoding parameters is available, the first nodemay select default precoding parameters for the data transmission. Ineither case, the second node must be notified which precoding parametersare used, in order to either receive and process received signalsproperly when being a receiving node, or transmit data accordingly whenbeing a sending node.

In a following step 304, information bits in the precoding informationfield(s) in the control message are therefore encoded according to thedetermined/selected precoding parameters, using values in the TBSfields. In this way, the various constituent parts in the precodinginformation field(s) are jointly encoded and the values set in thepayload size related TBS fields will affect the encoding. Thereby, thebits in the precoding information field(s) can imply different precodingrelated information or messages depending on the values in the TBSfields, without increasing the overall signalling overhead. Theavailable signalling space in the precoding information field(s) is alsoutilised more efficiently, as compared to the prior art solutions. It isalso possible to compress the space for precoding information in themessage, e.g. by omitting a HARQ flag field which can be indicated bythe TBS field values instead, which will be described in more detailbelow.

In addition to the above encoding of information bits in the precodinginformation field(s) according to selected precoding parameters,conventional channel coding of the control message may further beemployed, which is however outside the scope of the present invention.In a final step 306, the control message, containing the TBS encodedbits in the precoding information field(s), is sent to the second node.

A corresponding procedure will be executed in the second node to obtainthe precoding information in the control message from the first node.Firstly, the control message is received including precoding parametersdetermined by the first node. Secondly, the precoding parameters aredetected by decoding control information bits in at least one precodinginformation field of the control message by means of values in payloadsize related TBS fields in the control message. The TBS field values arethen used to interpret the control information bits in the precodinginformation field(s).

FIG. 4 is a block diagram illustrating an apparatus in a first node 400for conveying precoding information in a control message to a secondnode 402. The first node 400 may be a data sending node and the secondnode 402 may be a data receiving node, or vice versa. As in the exampleof FIG. 3 described above, the control message contains information thatdescribes properties of an associated wireless data transmission betweenthe nodes employing spatial multiplexing and precoding for sendingcodewords corresponding to transport blocks in the data transmission.The control message is configured to contain one or more precodinginformation fields and TBS fields, or generally payload size relatedfields. The first node apparatus in FIG. 4 comprises functional units400 a-d basically adapted to support the procedure of FIG. 3.

The first node apparatus in FIG. 4 comprises a precoding determiningunit 400 a adapted to determine precoding parameters P for the datatransmission with the second node, e.g. according to regular proceduresand optionally based on feedback reports F received from the second node402 if the first node is the one sending data. Unit 400 a provides theprecoding parameters P to a control message encoding unit 400 b, whichis adapted to encode information bits in the precoding informationfield(s) in the message based on values set in the TBS fields inaccordance with the determined precoding parameters P, basically asdescribed above for step 304.

The first node apparatus 400 further comprises a control message sendingunit 400 c, which is adapted to send the control message M with TBSencoded bits in the precoding information field(s) to the second node402. If the first node 400 is the data sending node, the determinedprecoding parameters P are also provided to a data transmitter 400 d foruse during transmission of data D to the second node 402. On the otherhand, if the second node 402 is the data sending node, the precodingparameters P received from the first node 400 are provided to acorresponding data transmitter in the second node 402, not shown, foruse during transmission of data to the first node 400. Depending on theimplementation, unit 400 a may provide the parameters P to unit 400 beither directly or via the data transmitter 400 d or via any otherfunctional unit not shown here, and this embodiment is generally notlimited in this respect.

An apparatus is also provided in the second node 402 for obtaining theprecoding information in the control message M from the first node 400.The second node apparatus 402 comprises a control message receiving unit402 a adapted to receive the control message M including the precodingparameters determined by the first node, and a control message decodingunit 402 b. The latter is adapted to detect the precoding parameters bydecoding control information bits in at least one precoding informationfield of the control message by means of values in the payload sizerelated TBS fields in the control message. The control message decodingunit 402 b uses the TBS field values to interpret the controlinformation bits in the precoding information field(s).

It should be noted that FIG. 4 merely illustrates the functional units400 a-d and 402 a,b in a logical sense, while the above-describedfunctions can be implemented in practice using any suitable software andhardware, without departing from the present invention. Typically, bothdata and control messages are communicated by means of the same physicaltransmission equipment in each node including transceiver and antennaparts.

It will now be described in more detail how the encoding of informationbits in precoding information fields in the control message, based onvalues set in the TBS fields, can be executed in practice by means ofsome exemplary embodiments. The following sections describe how bits inthe precoding information fields and MIMO related fields in the PDCCHmessage used in LTE may be encoded for support of precoding and spatialmultiplexing mode when transmitting data from a sending node to areceiving node.

In FIG. 5, a table is shown with different information fields in a PDCCHmessage configured in accordance with the current proposed LTE standardto indicate various parameters relevant for transmission, which a datasending node is obliged to send to a data receiving node. The firstcolumn indicates a field order number, the second column indicates thefield contents, and the third column specifies how many bits areavailable in each PDCCH field. The first column also contains anasterisk (*) for those fields carrying information needed when MIMO isemployed. The shown PDCCH message can provide for the simultaneoustransmission of a maximum of two codewords 1 and 2.

In more detail, the numbered PDCCH fields in FIG. 5 relate to: 1)resource allocation with a varying number×of bits depending on thesystem bandwidth, 2) transmission power control with 2 bits, 3) a firstTBS 1 for codeword 1 with 5 bits, 4) a second TBS 2 for codeword 2 with5 bits, 5) New Data Indicator NDI 1 and Redundancy Version RV 1 forcodeword 1 with 3 bits, 6) NDI 2 and RV 2 for codeword 2 with 3 bits, 7)a HARQ process identity with 3 bits, 8) Precoder Matrix Information PMIwith 4 bits indicating which precoder matrix is actually used, 9)precoder confirmation with 1 bit that can be “1” if recommendedtransmission rank and precoder matrix are used or “0” if another rankand matrix are used, 10) transmission Rank Indicator RI with 2 bits, and11) HARQ swap flag with 1 bit. It should be noted that the parametersPMI and RI above may also be referred to as TPMI (Transmission PMI) andTRI (Transmission RI), respectively.

According to FIG. 5, the PDCCH fields 4, 6 and 8-11 all contain “MIMOrelated” information, i.e. information communicated only when MIMO isemployed, and fields 8-11 specifically contain precoding information.Hence, a total of 16 bits in the message can be used for MIMOinformation out of which 8 bits relate to precoding. Moreover, there aretwo TBS fields allowing for two different TBS values TBS 1 and TBS 2 toindicate the transport block size used for up to two correspondingcodewords.

According to different embodiments, the TBS values for the two codewords1 and 2 can be selected to also encode the precoding information in theshown PDCCH format as follows. The TBS values TBS 1 and TBS 2 determinethe interpretation of the precoder information bits in fields 8-11.Thus, the TBS fields indicate a payload size pair (TBS1, TBS2)corresponding to the payload size of a first transport block and asecond transport block, respectively. The payload size pair (TBS1, TBS2)for the two codewords signals or indicates the following:

-   -   (TBS, 0): One codeword is enabled and transmitted with size TBS        while the other codeword is disabled.    -   (TBS1, TBS2): Two codewords are enabled and transmitted        simultaneously with size TBS1 and TBS2, respectively.

In addition to indicating the number of transmitted codewords, thesesignalling options can also determine the interpretation of the precoderinformation bits. This provides for compression of the size of thecontrol message and may even allow for support of additionalfunctionality such as, e.g., transmission rank override for precoderconfirmation when frequency-selective precoding is employed. In oneembodiment, the transmission rank override can be indicated by usingspecified columns of all recommended precoders in the latest feedbackreport conveyed from the receiving node to the sending node. Theinformation types RI, PMI and precoder confirmation in fields 10, 8 and9, respectively, can be jointly encoded as specified below for twodifferent MIMO transmission cases. It should be noted that the differentPMI values below refer to specific predefined precoding matrices, e.g.in a known codebook.

1. MIMO with Two Transmit Antennas (2 Tx MIMO)

When transmission rank=1 and one codeword is enabled by signalling(TBS,0), 3 bits can be used to signal one of 6 different possibleprecoding related messages with the precoding information, including:

-   -   a) 4 messages with RI=1 combined with PMI=0, 1, 2 or 3,    -   b) 1 message with RI=1 combined with “precoder report confirmed,        use 1^(st) precoder column”, and    -   c) 1 message with RI=1 combined with “precoder report confirmed,        use 2^(nd) precoder column”.

When transmission rank=2 and two codewords are enabled by signalling(TBS1, TBS2), 2 bits can be used to signal one of 3 different possibleprecoding related messages, including:

-   -   d) 2 messages with RI=1 combined with PMI=0 or 1, and    -   e) 1 message with RI=2 combined with “precoder report        confirmed”.        2. MIMO with Four Transmit Antennas (4 Tx MIMO)

When transmission rank=1 or 2 and one codeword is enabled by signalling(TBS,0), 6 bits can be used to signal one of 34 different possibleprecoding related messages including:

-   -   a) 16 messages with RI=1 combined with PMI=0, 1, 2, . . . or 15,    -   b) 1 message with RI=1 combined with “precoder report        confirmed”,    -   c) 16 messages with RI=2 combined with PMI=0, 1, 2, . . . or 15,        and    -   d) 1 message with RI=2 combined with “precoder report        confirmed”.

When transmission rank=2, 3 or 4 and two codewords are enabled bysignalling (TBS1,TBS2), 6 bits can be used to signal one of 51 differentpossible precoding related messages including:

-   -   e) 16 messages with RI=2 combined with PMI=0, 1, 2, . . . or 15,    -   f) 1 message with RI=2 combined with “precoder report        confirmed”,    -   g) 16 messages with RI=3 combined with PMI=0, 1, 2, . . . or 15,    -   h) 1 message with RI=3 combined with “precoder report        confirmed”.    -   i) 16 messages with RI=4 combined with PMI=0, 1, 2, . . . or 15,    -   j) 1 message with RI=4 combined with “precoder report        confirmed”.

In this way, the number of bits needed to convey the precoderinformation above can be reduced in the PDCCH message since the TBSvalues control and limit the choice of possible precoding messages. Itshould be noted that when 1 codeword is transmitted and RI=2 for 4 TxMIMO, as shown in the examples 2c) and 2d) above, one codeword istransmitted on two layers, such as when CW1 is transmitted on L1 and L2in FIG. 2 c. Furthermore, for 4 Tx MIMO in the examples 2a)-j) above,the precoder column subset is implicitly known and can be derivedthrough the codeword to layer mappings shown in FIG. 2 a-e.

It is also possible to omit field 11 of FIG. 5 normally containing theHARQ swap flag which indicates the mapping from HARQ processes tocodewords. In addition to indicating the number of transmittedcodewords, the TBS values in fields 3 and 4 of FIG. 5 can be furtherused to indicate the HARQ process mapping according to the following:

-   -   (TBS, 0): HARQ process 1 is mapped to codeword 1 which is        transmitted,    -   (0, TBS): HARQ process 2 is mapped to codeword 1 which is        transmitted, and    -   (TBS1, TBS2): Codewords 1 and 2 are both transmitted where HARQ        process 1 is mapped to codeword 1 and HARQ process 2 is mapped        to codeword 2.

The above allows selection of which HARQ to transmit when thetransmission rank=1. Alternatively, when a fixed HARQ process tocodeword mapping is used, the TBS fields can be further used toindicate:

-   -   (TBS, 0): Codeword 1 is transmitted,    -   (0, TBS): Codeword 2 is transmitted, or    -   (TBS1, TBS2): Codewords 1 and 2 are both transmitted.

The latter embodiments above would effectively introduce several newcodeword to layer mappings, thereby providing a more flexible choice ofwhich layer to use in the transmission. For example, when transmissionrank=2, it would be possible to transmit a single codeword using twolayers, e.g. as shown in FIG. 2 c.

The exemplary embodiments described above can be used to improve theefficiency for encoding precoder related information, e.g. in a controlmessage from a base station to a user terminal to control an associatedwireless data transmission either on the downlink or the uplink.Signalling overhead can thus be saved by decreasing the control channelbits, while at the same time additional functionality can be introducedfor supporting rank override and for transmission of a single codewordusing two-layer spatial multiplexing. It should be noted that theabove-described encoding of information bits in the precodinginformation field(s) to indicate that a codeword is enabled/disabled,may also be jointly based on values in other fields in the controlmessage, e.g. the NDI/RV fields in FIG. 5, in addition to values set inthe TBS fields. The present invention is thus not limited in thisrespect.

While the invention has been described with reference to specificexemplary embodiments, the description is in general only intended toillustrate the inventive concept and should not be taken as limiting thescope of the invention. For example, although the concepts of LTE, OFDM,MIMO, transport blocks, codewords, layers, feedback reports,transmission rank and channel rank have been used when describing theabove embodiments, any other similar suitable terms, parameters,mechanisms and standards may basically be used to accomplish thefunctions described herein. The present invention is generally definedby the following independent claims.

1. A method in a first node of conveying precoding information in acontrol message to a second node, the control message containinginformation that describes properties of an associated wireless datatransmission between the first and second nodes employing spatialmultiplexing and precoding information for sending codewordscorresponding to transport blocks in said wireless data transmission,comprising the steps of: determining precoding parameters to be includedin said control message to the second node, encoding control informationbits in at least one precoding information field of the control messageaccording to the determined precoding parameters by means of values inpayload size related fields in the control message, wherein said payloadsize related field values are set to determine the interpretation ofsaid control information bits in the precoding information field(s), andsending said control message with said precoding information and payloadsize related fields to the second node.
 2. A method according to claim1, wherein bits in the precoding information field(s) of the controlmessage imply different predefined sets of precoding informationmessages depending on the set payload size related field values.
 3. Amethod according to claim 1, wherein the precoding information comprisesany one or both of: a transmission rank indicating the number ofparallel layers or data streams being simultaneously used for theassociated data transmission, and at least one selected precoder matrixused for adapting signals transmitted from multiple antennas at the nodesending said data.
 4. A method according to claim 1, wherein said datais transmitted from the first node to the second node and said precodingparameters are determined based on current channel properties indicatedin a feedback report received from the second node, and/or by the amountof retransmissions due to decoding errors at the second node.
 5. Amethod according to claim 1, wherein the second node is a terminal andthe first node is a base station ordering the terminal to transmituplink data according to the conveyed precoding information.
 6. A methodaccording to claim 1, wherein said payload size related fields indicatea payload size pair corresponding to the payload size of a firsttransport block and a second transport block, and said payload size pairis set to indicate: that a first codeword is enabled while a secondcodeword is disabled, or that two codewords are enabled.
 7. A methodaccording to claim 6, wherein said payload size pair also determines theinterpretation of precoder information bits in the control message tosupport transmission rank override for precoder confirmation whenfrequency-selective precoding is employed.
 8. A method according toclaim 1, the control message being a physical downlink control channel(PDCCH) message with precoding related information fields, saidprecoding related information corresponding to a “Rank Indicator RI” or“Transmission Rank Indicator TRI”, a “Precoder Matrix Indicator PMI”,and/or precoder confirmation, wherein these parts in said precodingrelated information are jointly encoded.
 9. A method according to claim1, wherein the payload size related field values are used to indicate ahybrid automatic repeat request (HARQ) process to codeword mapping, andwherein the payload size related fields values are set to indicate: thatHARQ process 1 is mapped to codeword 1 which is transmitted, that HARQprocess 2 is mapped to codeword 1 which is transmitted, or thatcodewords 1 and 2 are both transmitted.
 10. A method according to claim1, wherein a fixed hybrid automatic repeat request (HARQ) process tocodeword mapping is used, and wherein the payload size field relatedvalues are set to indicate: that codeword 1 is transmitted, thatcodeword 2 is transmitted, or that codewords 1 and 2 are bothtransmitted.
 11. An apparatus in a first node, for conveying precodinginformation in a control message to a second node, the control messagecontaining information that describes properties of an associatedwireless data transmission between the first and second nodes employingspatial multiplexing and precoding information for sending codewordscorresponding to transport blocks in said wireless data transmission,comprising: a precoding determining unit adapted to determine precodingparameters to be included in said control message to the second node, acontrol message encoding unit adapted to encode control information bitsin at least one precoding information field of the control messageaccording to the determined precoding parameters by means of values inpayload size related fields in the control message, wherein the payloadsize related field values are set to determine the interpretation ofsaid control information bits in the precoding information field(s), anda control message sending unit adapted to send said control message withsaid precoding information and payload size related fields to the secondnode.
 12. An apparatus according to claim 11, wherein bits in theprecoding information field(s) of the control message imply differentpredefined sets of precoding information messages depending on the setpayload size related field values.
 13. An apparatus according to claim11, wherein the precoding information comprises any one or both of: atransmission rank indicating the number of parallel layers or datastreams being simultaneously used for the associated data transmission,and at least one selected precoder matrix used for adapting signalstransmitted from multiple antennas at the node sending said data.
 14. Anapparatus according to claim 11, further comprising a signal transmitteradapted to transmit said data to the second node using the determinedprecoding parameters, wherein the precoding determining unit is furtheradapted to determine said precoding parameters based on current channelproperties indicated in a feedback report received from the second node,and/or by the amount of retransmissions due to decoding errors at thesecond node.
 15. An apparatus according to claim 11, wherein the firstnode is a base station and the second node is a terminal, the basestation ordering the terminal to transmit uplink data according to theconveyed precoding information.
 16. An apparatus according to claim 11,wherein said payload size related fields indicate a payload size paircorresponding to the payload size of a first transport block and asecond transport block, and the control message encoding unit is furtheradapted to set said payload size pair to indicate: that a first codewordis enabled while a second codeword is disabled, or that two codewordsare enabled.
 17. An apparatus according to claim 16, wherein saidpayload size pair also determines the interpretation of precoderinformation bits in the control message to support transmission rankoverride for precoder confirmation when frequency-selective precoding isemployed.
 18. An apparatus according to claim 11, the control messagebeing a physical downlink control channel (PDCCH) message with precodingrelated information fields, said precoding related informationcorresponding to a “Rank Indicator RI” or “Transmission Rank IndicatorTRI”, a “Precoder Matrix Indicator PMI”, and/or precoder confirmation,wherein the control message encoding unit is further adapted to jointlyencode these parts in said precoding related information.
 19. Anapparatus according to claim 11, wherein the control message encodingunit is further adapted to use the field values to indicate a hybridautomatic repeat request (HARQ) process to codeword mapping, and whereinthe payload size related values are set to indicate: that HARQ process 1is mapped to codeword 1 which is transmitted, that HARQ process 2 ismapped to codeword 1 which is transmitted, or that codewords 1 and 2 areboth transmitted.
 20. An apparatus according to claim 11, wherein afixed hybrid automatic repeat request (HARQ) process to codeword mappingis used and the control message encoding unit is further adapted to usethe payload size related field values to indicate: that codeword 1 istransmitted, that codeword 2 is transmitted, or that codewords 1 and 2are both transmitted.
 21. A method in a second node of obtainingprecoding information in a control message from a first node, thecontrol message containing information that describes properties of anassociated wireless data transmission between the first and second nodesemploying spatial multiplexing and precoding information for sendingcodewords corresponding to transport blocks in said wireless datatransmission, comprising the steps of: receiving said control messageincluding precoding parameters determined by the first node, anddetecting said precoding parameters by decoding control information bitsin at least one precoding information field of the control message bymeans of values in payload size related fields in the control message,wherein said values are used to interpret said control information bitsin the precoding information field(s).
 22. A method according to claim21, wherein the second node receives said data from the first node andsaid precoding parameters have been determined based on current channelproperties indicated in a feedback report sent from the second node,and/or by the amount of retransmissions due to decoding errors at thesecond node.
 23. A method according to claim 21, wherein the first nodeis a base station and the second node is a terminal being ordered by thebase station to transmit uplink data according to the conveyed precodinginformation.
 24. A method according to claim 21, wherein bits in theprecoding information field(s) of the control message imply differentpredefined sets of precoding information messages depending on the setpayload size related field values.
 25. A method according to claim 21,wherein the precoding information comprises any one or both of: atransmission rank indicating the number of parallel layers or datastreams being simultaneously used for the associated data transmission,and at least one selected precoder matrix used for adapting signalstransmitted from multiple antennas at the node sending said data.
 26. Amethod according to claim 21, wherein said payload size related fieldsindicate a payload size pair corresponding to the payload size of afirst transport block and a second transport block, and said payloadsize pair is set to indicate: that a first codeword is enabled while asecond codeword is disabled, or that two codewords are enabled.
 27. Amethod according to claim 26, wherein said payload size pair alsodetermines the interpretation of precoder information bits in thecontrol message to support transmission rank override for precoderconfirmation when frequency-selective precoding is employed.
 28. Amethod according to claim 21, the control message being a physicaldownlink control channel (PDCCH) message with precoding relatedinformation fields, said precoding related information corresponding toa “Rank Indicator RI” or “Transmission Rank Indicator TRI”, a “PrecoderMatrix Indicator PMI”, and/or precoder confirmation, wherein these partsin said precoding related information are jointly encoded.
 29. A methodaccording to claim 21, wherein the payload size related field values areused to indicate a HARQ process to codeword mapping, and wherein thepayload size related fields values are set to indicate: that HARQprocess 1 is mapped to codeword 1 which is transmitted, that HARQprocess 2 is mapped to codeword 1 which is transmitted, or thatcodewords 1 and 2 are both transmitted.
 30. A method according to claim21, wherein a fixed hybrid automatic repeat request (HARQ) process tocodeword mapping is used and the payload size related field values areused to indicate: that codeword 1 is transmitted, that codeword 2 istransmitted, or that codewords 1 and 2 are both transmitted.
 31. Anapparatus in a second node of obtaining precoding information in acontrol message from a first node, the control message containinginformation that describes properties of an associated wireless datatransmission between the first and second nodes employing spatialmultiplexing and precoding information for sending codewordscorresponding to transport blocks in said wireless data transmission,comprising: a control message receiving unit adapted to receive saidcontrol message including precoding parameters determined by the firstnode, and a control message decoding unit adapted to detect saidprecoding parameters by decoding control information bits in at leastone precoding information field of the control message by means ofvalues in payload size related fields in the control message, using saidpayload size related field values to interpret said control informationbits in the precoding information field(s).
 32. An apparatus accordingto claim 31, wherein the second node is adapted to receive said datafrom the first node and said precoding parameters have been determinedbased on current channel properties indicated in a feedback report sentfrom the second node, and/or by the amount of retransmissions due todecoding errors at the second node.
 33. An apparatus according to claim31, wherein the first node is a base station and the second node is aterminal being ordered by the base station to transmit uplink dataaccording to the conveyed precoding information.
 34. An apparatusaccording to claim 31, wherein bits in the precoding informationfield(s) of the control message imply different predefined sets ofprecoding information messages depending on the set payload size relatedfield values.
 35. An apparatus according to claim 31, wherein theprecoding information comprises any one or both of: a transmission rankindicating the number of parallel layers or data streams beingsimultaneously used for the associated data transmission, and at leastone selected precoder matrix used for adapting signals transmitted frommultiple antennas at the node sending said data.
 36. An apparatusaccording to claim 31, wherein said payload size related fields indicatea payload size pair corresponding to the payload size of a firsttransport block and a second transport block, and said payload size pairis set to indicate: that a first codeword is enabled while a secondcodeword is disabled, or that two codewords are enabled.
 37. Anapparatus according to claim 36, wherein said payload size pair alsodetermines the interpretation of precoder information bits in thecontrol message to support transmission rank override for precoderconfirmation when frequency-selective precoding is employed.
 38. Anapparatus according to claim 31, the control message being a physicaldownlink control channel (PDCCH) message with precoding relatedinformation fields, said precoding related information corresponding toa “Rank Indicator RI” or “Transmission Rank Indicator TRI”, a “PrecoderMatrix Indicator PMI”, and/or precoder confirmation, wherein these partsin said precoding related information are jointly encoded.
 39. Anapparatus according to claim 31, wherein the payload size related fieldvalues are used to indicate a hybrid automatic repeat request (HARQ)process to codeword mapping, and wherein said payload size pair is setto indicate: that HARQ process 1 is mapped to codeword 1 which istransmitted, that HARQ process 2 is mapped to codeword 1 which istransmitted, or that codewords 1 and 2 are both transmitted.
 40. Anapparatus according to claim 31, wherein a fixed hybrid automatic repeatrequest (HARQ) process to codeword mapping is used and the payload sizerelated field values are used to indicate: that codeword 1 istransmitted, that codeword 2 is transmitted, or that codewords 1 and 2are both transmitted.